Fluidic Catalytic Cracking units are playing an increasingly important role in the production of propylene. The use of Y-zeolites in conjunction with pentasil zeolites, in particular, ZSM-5, has led to increases in the amount of propylene which can be produced from petroleum fraction feedstocks such as deep cut gas oil, vacuum gas oil, thermal oil, residual oil, cycle stock, whole top crude, tar sand oil, shale oil, synthetic fuel, heavy hydrocarbon fractions, such as those derived from the destructive hydrogenation of coal, tar, pitches, asphalts, hydrotreated feedstocks derived from any of the foregoing, and the like. Such Y-zeolites are generally doped or exchanged with rare-earth metals such as Lanthanum, Cerium, Neodymium and Praseodymium. These “rare-earth exchanged” zeolites are preferred because they have a high percentage of conversion which, it has been thought, is necessary in order to obtain high amounts of gasoline fraction which is generally thought to be necessary in order to maximize the amount of propylene which is produced by further cracking by the ZSM-catalyst.
However, such rare earth zeolites have reached a maximum in the amount of propylene that a given combination of rare earth-exchanged zeolites can generate in the copresence of ZSM-type zeolites. It has heretofore been thought that other types of catalyst systems would have to be employed in order to get further gains in the amount of propylene with respect to the amounts produced by Y-zeolite/ZSM catalytic systems. While other types of dopants have been tried, many reduced the rate of conversion by the Y-zeolite, giving rise to overall efficiency considerations.